Field of the Invention
The invention relates to a method for producing a hard metal granulate. The process involves wet milling of the hard material and binding metal components that are desired in the finished granulate and the formation of a sprayable slurry using pure water as a liquid phase. The slurry is converted to granular form in a spray tower through spray drying in a gas stream with a gas entry temperature in the range from 160° to 220° C. and a gas exit temperature ranging from 85° to 130° C. The spray tower has a cylindrical segment and a conical segment.
Molded parts made of hard metal alloys are produced by pressing and sintering powdered base materials. In order to make them easier to process, the fine-grained base powder of the hard metal alloys with a mean particle size in the range of only several microns (μm) and often smaller are converted to granular form, i.e. in the most ideal spherical form possible with a mean particle size of at least 90 μm. This is accomplished by milling the hard material and binding metal components in a liquid medium to form a finely dispersed mixture which takes the form of a slurry. When coarser-grained starting powders are used, this step also involves milling the starting powders, whereas the slurry is merely homogenized when fine-grained starting powders are used. The liquid protects the powder particles against fusion and prevents them from oxidizing during the milling process.
Suitable milling systems used almost exclusively in present day processing are agitator ball mills known as attritors. There, the material to be milled is set in motion together with hard metal balls by a multiple-blade agitator arm inside a cylindrical container. A pressing aid, e.g. paraffin, can be introduced to the slurry produced through the liquid-enhanced milling process, if appropriate. The addition of a pressing aid is necessary especially in cases where the finished granulate is pressed in compacting dies into the desired form.
The pressing aid gives the granulate better compression properties during the pressing process and also enhances its flow characteristics, which facilitates the filling of compacting dies. If the finished hard metal granulate is to be further processed in an extruder press, no pressing aid is normally added to the slurry. The slurry is brought to a sprayable consistency, then dried and granulated simultaneously in a spray drying system. In this process, the slurry is sprayed through a nozzle positioned inside the spray tower. A stream of hot gas dries the airborne spray droplets, which then precipitate as granulate in the form of small granules or beads in the lower conical segment of the spray tower, from where it can then be removed. In the hard metal industry, such organic solvents as acetone, alcohol, hexane or heptane are still used almost exclusively in the milling and pressing of slurries today. These solvents are used in concentrated form or diluted only slightly with water.
Because all of these solvents are highly flammable and volatile, attritors and spray drying systems must be designed as explosion-resistant units, which requires considerable engineering design input and thus generates high costs. In addition, the materials must be dried in an inert gas atmosphere, ordinarily nitrogen, in the spray tower.
All of the above-mentioned solvents are also environmental pollutants and are subject to substantial evaporation loss, despite the use of recycling measures, due to their high volatility.
Spray towers in spray drying systems used in the hard metal industry are designed with a cylindrical upper segment and a conical, downward pointing lower segment and ordinarily operate in a countercurrent mode in accordance with the fountain principle, i.e. the sprayer lance is positioned in the center of the lower segment of the spray tower and sprays the slurry under high pressure (12-24 bar) upward in the form of a fountain. The gas stream which dries the sprayed droplets flows into the drying chamber from above, counter to the travel direction of the sprayed droplets, and escapes from the spray tower in the upper third portion of the conical, downward pointing segment below the spray lance. In this way, the droplets are first conveyed upward and then pulled downward by the force of gravity and the opposing stream of gas. In the course of the drying cycle, the droplets are transformed into a compact granulate with a low residual moisture content. As they fall to the floor of the spray tower, they automatically trickle down through the conical, downward pointing lower segment to the central discharge outlet.
Because the flight pattern of the sprayed droplets takes them first upward and then down, the distance traveled by the droplets during drying is equivalent to that of spray towers that operate with co-current downward streams of sprayed slurry and drying gas, but the process requires almost fifty percent less tower height. This results in a more compact spray tower construction.
Spray towers in practical use which operate with countercurrents on the basis of the fountain principle have a cylindrical segment measuring between 2 and 9 m in height with a height to diameter ratio of between 0.9 and 1.7, whereas spray towers which operate in a co-current mode with top-down gas and sludge flow are equipped with a cylindrical segment measuring between 5 and 25 m in height with height to diameter ratio ranging from 1 to 5.
In this specification, the general term “hard metal” is intended to also encompass so-called cermets, a special group of hard metals, which ordinarily contain hard materials with nitrogen.
U.S. Pat. No. 4,070,184 describes a process for producing a hard metal granulate involving milling and spray drying wherein pure water is used instead of organic solvents for milling and production of the sprayable slurry. The use of water as a liquid phase eliminates the need to construct attritors and spray drying systems as explosion-resistant units, which helps to reduce costs. In spray drying, air may be used instead of inert gas as a drying medium. Moreover, eliminating the use of organic solvents entirely rules out health risks posed by solvent vapors.
The major disadvantage of this process is that the use of pure water and air results in increased impairment of powder quality through oxidation. Extremely fine-grained hard metal powders with a mean particle size of 0.5-0.6 μm, which correlates on the basis of BET measurement to a surface area of 1.6-3.2 m2/g, which is used for many types of hard metal grades today, are highly susceptible to oxidation due to their large surface area and thus cannot be produced using this process. Even for hard metal powders with a larger mean particle size of 1 μm and slightly less and thus a considerably smaller surface area—the smallest standard particle sizes in common use at the time the US patent was registered, it was necessary to reduce susceptibility to oxidation by adding a long-chain polyglycol to the slurry immediately prior to spray drying. Such polyglycols, which also make the granulates more compactable, completely enclose the powder particles and thus largely prevent oxidation of the particles during spray drying.
The disadvantage of this process is that polyglycols of this type exhibit unfavorable vaporizing behavior during sintering of the pressed powder. Complete vaporization occurs only at temperatures between 250° and 300° C., which, due to vaporization over a broad temperature range, can cause the part to crack or form fissures.